If you search on here this is some old heated debates about this very subject that are filled with great information.
To answer your question quickly though, it all depends on the individual and their diet/training styles. Their are many people who do well with simple carbs like dextrose, complex carbs like oats, or high molecular weight carbs like Waxy Maize; but there are also people who hate one of those for postworkout. Personally I always found dextrose to work best for me, but now that I am on a carb restricted diet I utilize PeptoPro to induce an insulin spike without the use of carbohydrates. This way I get a good hormonal environment post workout but I can stay within my dietary restrictions. If you want more info on PeptoPro shot me a pm and I will explain it more.
Hope this helps

You want the insulin spike after your workout. The insulin spike send the sugars to replenish your glycogen stores that have just been drained and the amino acids of the proteins will begin the anabolic process of protein synthesis. Most people use Waxy Maize Starch (usually just called WMS around here), a combination of Maltodextrin/Dextrose or something like gatorade powder as their carb source. You want to avoid fat in your post-workout shakes. Not only will it be available for storage due to the insulin spike, but it will also slow the digestion of the protein, and you are looking for the fastest digestion possible in your Post-WO shake.

Edit:Pmiller beat me to it..... hope that gives you some guidance tho.